Tilman Sauer is an historian of physics specializing in general relativity, the foundations of physics, and the philosophy of quantum mechanics. He was a contributing editor for volumes 3 and 4 of the Collected Papers of Albert Einstein.

While carefully reading through some 1800 pages of Einstein's manuscripts that were shipped from Princeton to the Albert Einstein Archives in Jerusalem, Sauer found a few short lines about the Einstein-Podolsky-Rosen paradox that considered the result of a measurement of a two-valued spin component and the "knowledge at a distance" that comes into existence instantly for widely separate entangled particles.

Einstein was clearly responding to the 1952 suggestion of David Bohm that EPR measurements could be carried our more easily on the discrete values of electron spin than the continuous values of position and momentum in the original EPR paper. Bohm was hoping that such measurements could reveal the existence of "hidden variables."

Composite system of total spin 0.
1) The description is assumed to be complete.
2) A coupling of distant things is excluded.

If the spin of the subsystem I is measured along the x-axis, it is
found to be either 1 or −1 in that direction.

The average value of spins along an unmeasured direction is 0, new measurements project the spin randomly as 1 and -1

It then follows that
the spin of the subsystem II equals 0 along the y-direction.

If y-component is 1 for particle I, it is -1 for particle II (anti-correlated to conserve total spin zero)

But if
instead the spin of subsystem I is measured along the y-direction,
it follows that the spin of the subsystem II is equal to 1 or −1.

If there is no coupling, then the result of a measurement of the
spin of subsystem II may in no way depend on whether a measurement
was taken of subsystem I (or on what kind of measurement).

The two assumptions therefore cannot be combined.

If the description is not assumed to be complete for the individual
system, then that what is being described is not a single system
but an ensemble of systems. Then a measurement of subsystem I
amounts to the selection of a subensemble of the ensemble of the
total system. Then the prediction for a measurement of subsystem
II can depend on the choice of the measurement of subsystem
I.

Sauer says the following lines were written at the right margin of the page:

a) the description by the quantum theory is an incomplete one
with respect to the individual system, or
b) there is an immediate coupling of states of spatially separated
things.

This last line is what Erwin Schrödinger had told Einstein would be the case back in 1935, calling the particles "entangled."

While Einstein called this "action-at-a-distance" in 1935 (even for a single particle relative to its wave in 1927), and more dramatically called it "spooky action-at-a-distance" in 1948, we now know it is just "knowledge-at-a-distance." Neither particle "acts" on the other. They acquire their values simultaneously to conserve the total spin.